Many methods for the determination of phosphate ion have been demonstrated in the areas of gravimetric, titrimetric, and spectrophotometric analysis. Included in the methods available, as a minor technique, is the determination of phosphate with ion-selective electrodes. The obvious reason for the lack of use of electrodes is the poor selectivity of the phosphate electrodes constructed to date.
Presently known potentiometric membrane electrodes for anions display a selectivity pattern known as the Hoffmeister series. The well known Hoffmeister series ranks anions in order of decreasing hydrophobicity. It is commonly thought that electrode selectivity for anions is governed by the ability of the anion to enter the hydrophobic polymer membrane. Of course, this ability is greater for more hydrophobic anions and as a result a Hoffmeister series-type selectivity pattern is attained. The Hoffmeister series for several common anions is as follows:
perchlorate&gt;thiocyanate&gt;iodide&gt;nitrate&gt;bromide&gt;chloride&gt;bicarbonate&gt;phospha te&gt;carbonate&gt;sulfate
Hence, selectivity for conventional anion electrodes is quite good for perchlorate and thiocyanate, but it is extremely poor for phosphate and sulfate.
The first membrane electrode with a selectivity pattern different than the Hoffmeister series was that for carbonate (Rechnitz, Science 184 (1974) 1074; Wise, U.S. Pat. No. 3,723,281, 1973). This electrode utilizes trifluoroacetyl-p-butylbenzene as the membrane active component. A recent study indicates that this material acts as a neutral carrier for carbonate by complexing with carbonate at the electron-deficient carbon of the carbonyl group to form a hydrophobic carbonate dianion species (Meyerhoff, Anal. Chem. 59 (1987) 144). This electrode is employed in various commercial instruments for carbonate determinations because of its excellent selectivity.
A group of Russian Workers have reported, and patented, the use of dialkyltin dinitrates as a selective reagent for phosphate electrodes. (U.S.S.R. Pat. No. SU 721731, Mar. 15, 1980; Zarinski, V.A. et al., Electrochemcial Properties of Liquid Membranes Based On Dialkyltin(IV) Compounds In Phosphorus(V) Solutions, UDC 543.257.5:546.18, V.I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Academy of Sciences of the USSR, Moscow. Translated from Zhurnal Analiticheskoi Khimii, Vol. 35, No. 11, pp. 2137-2142, November, 1980; and Zarinski, V.A. et al., Dialkyltin(IV) Compounds as Active Components of the Liquid Membranes of Ion-Selective Electrodes in Arsenic(V) Solutions, UDC 543.257.1:546.19, V.I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Academy of Sciences of the USSR, Moscow. Translated from Zhurnal Analiticheskoi Khimii, Vol. 35, No. 11, pp. 2143-2148, November, 1980). Their electrode system responds only to high levels of phosphate (10-100 mM) and its selectivity properties have not been thoroughly explored. In addition, their system apparently must use a liquid membrane electrode configuration, and the membrane active material is dioctyltin dinitrate. Other work based on substituted tin compounds has been reported, but no electrodes with a selective response to phosphate have been reported.
There is a tremendous need for a phosphate selective electrode in nearly all areas of science and technology. This need is based on the overall importance of phosphate and the current lack of suitable analytical procedures for phosphate determination. Phosphate levels are important in a wide variety of situations based on the importance of phosphate in biochemical and physiological processes, the presence of phosphates in fertilizers and minerals, and the magnitude of phosphates in industrial water processing. Despite the overall significance of phosphate, the development of a selective electrode system for phosphate has not been successful. In a recent review of phosphate electrode attempts, Midgley (ISE Reviews, 8 (1986) 3) reports "None of the experimental designs has reached commercial production and the work so far does little to suggest that good selectivity can be achieved." This statement points out that none of the many attempts have successfully resulted in a selective phosphate system and that there is little promise that these strategies will be successful in the future.
Those concerned with these and other problems recognize the need for an improved phosphate selective membrane electrode.